This invention relates to an optical system for a copier with variable magnification capability and more particularly to such an optical system for projecting on a recording medium an image of an original document at a selected magnification by using a lens having a fixed focal length.
An optical system including many mirrors and a lens is used by an electrophotographic copier to form an image of a document on a recording medium such as a photosensitive body with a photoconductive surface. Such an optical system arranges its mirrors and lens in such a way that the optical path length a between the document surface and the lens is equal to the optical path length b between the lens and the image-forming surface of the recording medium when no magnification is desired (or magnification =1). If the lens has a fixed focal length denoted by f, the optical path lengths a and b are set such that a=b=2f. This is because a and b are related by the well-known lens formula f=ab/(a+b) and since magnification m is defined by b/a and hence a=b if the magnification is 1, the lens formula provides f=a/2=b/2 in this situation.
If it is desired to form an enlarged image on the recording medium or that m be greater than 1, this is achieved by moving the lens closer to the document surface and if reduction in size is desired it is likewise achieved by moving the lens closer to the recording medium. In other words, the lens must be moved to vary the magnification and, in addition, the mirrors disposed in the optical path must also be moved simultaneously in order to adjust the total optical path length (a+b) corresponding to the selected magnification. In prior art optical systems, the lens and the mirrors are adapted to move in coordination with respect to each other according to the variable magnification. Mechanisms for moving the lens and the mirrors according to the selected magnification are generally very complicated and occupy a large space. Some systems use independent driving units for the lens and the mirrors but not only are such systems costly but each driving unit must also be accurately driven in coordination according to the magnification. Japanese Patent Publication Koho No. 59-69749 entitled "Optical System for Electrophotographic Copier" disclosed a system using a single driver unit with which a lens and mirrors are moved together according to the selected magnification. In this system, the lens is moved to change the magnification by using a wire driven by a motor while the mirrors are moved by a planar cam connected to the same motor by means of a connector. If the range in which the magnification is variable is large, however, the planar cam and the like becomes complicated in structure and large in size. Thus, this technology can be applicable only to systems for which the magnification is not varied widely.